Coreless transformers, which may be composed of a bottom coil and a top coil, provide an attractive option for conventional CMOS process flow to fabricate products with bi-directional signal flow as used for controlling power switches at high voltage (for example, greater than 200 volts) in DC-DC converters and control of electric drives. The dielectric isolation strength to sustain the high operating voltage and robustness against very high voltage surges in particular, may be one of the limiting reliability problems. Frequently the high voltage robustness of such products may be much lower than expected from bulk material properties and the isolation distances defined by the thickness of the isolating layers and the layout and the product. After a dielectric breakdown a low ohmic conductive path may be formed between the top coil (which may be a point of high potential) and the guard ring (which may be at ground potential). Since high voltage isolation properties (for example, greater than 6,000 volts) are difficult to study experimentally, little is known about processing and environmental conditions affecting the dielectric isolation strength.